1. Field of the Invention
This invention resides in the field of time-delayed, automatic shut-off means for valves.
2. Prior Art
Generally, automatic shut-off means for valves such as exemplified by U.S. Pat. Nos. 2,181,581 (Fraser); 2,710,736 (Miller) 3,065,948 (Nolan) and 3,342,448 (Parkison) have characterized the prior art in this area.
All of the aforementioned patents, incorporate a time-delay mechanism which utilizes, as the metered liquid, the same liquid as controlled by the valve whose time-delayed closing is regulated by the dashpot mechanism.
In most applications, this liquid will be water. Use of water as the dashpot metering liquid typically results in minerals being deposited in and about the passageway through which the liquid is metered.
At the very least, the mineral deposits will restrict the effective diameter of the liquid metering passageway thereby increasing the time delay. Eventually, the metering passageway becomes plugged rendering the dashpot mechanism inoperative.
To remedy this situation, some of the prior art devices utilized a movable pin which moved in and out of the metering passageway to clear it of the restricting mineral deposits.
However, even the use of a pin to create a so-called "self-cleaning orifice" is inadequate to effect a reliable dashpot device which uses the water from the water supply because within a very short period of time the mineralization will clog the liquid metering combination and thereby render it inoperative.
Further, the solid particles within the water will, if not filtered, be deposited within the dashpot chamber and either completely fill it, or at the very least, unduly limit the stroke of the dashpot piston. In either case, the time delay is significantly shortened or becomes effectively "zero."
In addition, the viscosity of water acts as a practical limit on the amount of time delay available in a dashpot device such as found in the aforementioned patents.
Utilizing a liquid for dashpot metering other than water in a water supply system obviously requires that the metering liquid be isolated from the water supply system. However, one of the problems encountered when using a liquid other than water as the dashpot liquid is that during the operation of the dashpot a small amount of the dashpot liquid is lost. Eventually, the dashpot liquid becomes exhausted to such an extent that the dashpot becomes inoperative.
Further, the liquid selected for use as the dashpot liquid must have a rather large heat capacity to preclude it from being converted into its gaseous phase when the dashpot mechanism is used with a hot water supply. Should this occur, the dashpot time-delay would be reduced and/or the fluid seals may be unable to adequately contain the metering fluid in its gaseous phase.